This invention relates to an improved process for the manufacture of superphosphoric acid.
Concentrated phosphoric acids are called superphosphoric acids if they have a P.sub.2 O.sub.5 content of between 68 and 78%, or even more, and contain quantities, which increase with the concentration, of acids obtained by condensation of two or more molecules of orthophosphoric acid with elimination of a water molecule. Industrially, superphosphoric acids are prepared which have a P.sub.2 O.sub.5 concentration close to 70% and which can go up to 75%.
It is well known that superphosphoric acid, for example at 70% strength, has numerous advantageous properties not possessed by the 54% strength phosphoric acid generally marketed. One of its most significant properties is its sequestering power for solids which would be insoluble in concentrated solution. This property is particularly significant for the transportation of the acid, by boat for example, since the problems caused by the formation of precipitates are eliminated. Furthermore, since the superphosphoric acid is more concentrated, the quantity of water to be transported is smaller.
An important application of superphosphoric acid is in the manufacture of liquid fertilizers.
The superphosphoric acids are prepared by concentration of wet-process phosphoric acid. Various manufacturing processes have been proposed; for example, it has been proposed to use submerged combustion or evaporation by hot gases. However, these types of concentration processes involve significant losses of P.sub.2 O.sub.5 and give rise to problems of pollution. Moreover, the capacity of a line for manufacturing according to these processes is small.
Various processes for evaporation under vacuum have been proposed, and studies have been carried out on evaporators with long vertical tubes, film evaporators and forced circulation evaporators.
An industrial process for evaporation under vacuum is known in which the wet-process phosphoric acid, optionally preheated, is introduced into an evaporator operating under vacuum, a fraction of the phosphoric acid which issues from the evaporator being recycled to the evaporator by way of a heat exchanger heated by steam under pressure or another heat-transfer fluid. A fraction of the phosphoric acid thus circulates in a closed loop. A second fraction of the phosphoric acid issuing from the evaporator is drawn off and then sent for storage after being cooled.
In this single-stage process, the entire phosphoric acid is heated to a temperature of at least 240.degree. C., which corresponds to the use of steam at an absolute pressure of 30 to 37 bars in the heat exchanger. At these temperatures, there exists no steel able to resist for more than 4 or 5 years the corrosion by the phosphoric acid or, more especially, the corrosion by the byproduct fluorinated derivatives. It is therefore necessary to replace all the components of the installation at least every 5 years, or about 20% depreciation of the installation every year. In view of the prices of the special steels used, maintenance of the installation is a high operating cost. To this high cost are to be added the production losses caused by frequent stoppage of the installation, and losses of P.sub.2 O.sub.5 resulting from the cleaning and emptying of the different parts of the installation during stoppages.
A process has also been proposed and described in French Pat. No. 2,083,999, Dec. 17, 1971, by Parkson Corporation, the inventorship being Einar Henry Palamson, for the manufacture of superphosphoric acid having a concentration calculated in P.sub.2 O.sub.5 of close to 70%, and which can go up to 75%, by evaporation of wet-process phosphoric acid having a concentration calculated in P.sub.2 O.sub.5 preferably between 40 and 54% under vacuum in an installation incorporating an evaporator under vacuum, into which is introduced the phosphoric acid to be concentrated, optionally preheated. A first fraction of acid is drawn off from this evaporator and recycled into the evaporator by way of a heat exchanger heated by a heat-transfer fluid.
According to this process, the concentration is carried out in two stages:
in the first stage, the phosphoric acid is concentrated in a first evaporator under vacuum until it has a P.sub.2 O.sub.5 content of at least 65%, the heating in the exchanger being performed by means of a heat-transfer fluid having a temperature of 170.degree. to 220.degree. C., a first fraction of the phosphoric acid being recycled into the first evaporator;
in the second stage, the second fraction of the phosphoric acid issuing from the first evaporator is conveyed into a second evaporator where it is concentrated to a concentration greater than 65%, the heating in a second exchanger being performed by means of a heat-transfer fluid at a temperture of 215.degree. to 225.degree. C., the heat-transfer fluid used in the first exchanger being at an absolute pressure of 8 to 16 bars and the heat-transfer fluid used in the second exchanger being at an absolute pressure of 20 to 30 bars. As an important aspect of this process, evaporation is conducted under boiling conditions in the heat exchangers, thereby resulting in considerable turbulence and at a high rate of heat transfer.
Such a process enables superphosphoric acid to be manufactured at high concentrations, but because of the turbulence phenomena in the exchangers, increases the risks of corrosion. Furthermore, since the concentration of the impurities in the phosphoric acid are higher, fouling is caused on the walls of the exchanger, which increases the maintenance expenses.